Quin 2 is an EGTA derivative which can be loaded into cells to permit measurement of intracellular free ionic calcium levels, based upon spectrofluorimetric changes when calcium ions are bound. We have applied the method (which was originally described by Tsien et al.) to measure free intracellular calcium levels in lymphocytes neutrophils and platelets from peripheral blood, splenocytes, thymocytes and bovine adrenal cells.